PROJECT SUMMARY Ion channels are multispan transmembrane proteins that transport ~106 to 107 ions per second across membranes. The human genome encodes ~400 different ion channel (1.5% of the genome) that control such diverse processes as fertilization, proliferation, development, learning and memory. Although it is well validated that ion channels are at the core of many diseases, approved drugs are available for only a small percentage of this protein class. A major bottleneck in expanding the palette of ion channels as potential drug targets is the fact that several ion channels are only poorly characterized with respect to their biological function and regulation. Subcellular compartmentalization of ion channels to organellar membranes such as endoplasmic reticulum, endosomes or primary cilia further complicates a detailed biophysical characterization. As a consequence, a significant number of ion channels qualify as dark matter of the human genome with unknown function. One such dark matter ion channel is PKD2-L2, one of three members of the polycystin channel family. Mutations in the other polycystin members cause a plethora of human diseases, ranging from congenital heart disease and laterality defects to cyst formation in multiple organs (liver, kidney and pancreas). Polycystin channels are enriched in primary cilia, antenna-shaped protrusions of the apical membrane but the molecular mechanisms by which polycystin channels are regulated and thus contribute to ciliary signaling remains poorly understood. The central goal of this project is to functionally characterize PKD2-L2 containing polycystin channels and determine how PKD2-L2 contributes to ciliary signaling. There are two specific aims. The first aim characterizes the biophysical properties of homomeric and heteromeric polycystin channels containing PKD2-L2. The second aim is to determine the subcellular localization of homomeric and heteromeric PKD2-L2 channels. The applicants’ preliminary observations include several novel unpublished methods. Completion of this project will be a critical step towards understanding the fundamental principles of ion channel signaling within primary cilia. Our long‐ term goal is to understand how dysregulation of ciliary ion channels cause human ciliopathies and establish PKD2-L2 as a therapeutic target.

Project Narrative Defects in electrical signaling within primary cilia are considered a primary cause for various human ciliopathies, ranging from cyst formation in kidney, liver and pancreas to congenital heart defects. The proposed work will characterize the key biophysical properties of the dark matter ion channel PKD2-L2 and will define the molecular mechanisms of channel activation. Knowledge generated in this proposal will shed light on the fundamental principles how different polycystin channels use primary cilia to sense the local environment convert this information into a cellular output.